1. Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating the same and more specifically to a semiconductor device using gallium nitride (GaN) semiconductor materials and a method for fabricating the same.
2. Description of the Related Art
Conventionally, gallium nitride (GaN) semiconductor materials have a large band gap energy as compared to gallium arsenide (GaAs) materials and have an excellent heat resistance property, so that they may be operated in high temperature. Due to such merits, developments of semiconductor devices using the GaN semiconductor materials and GaN/AlGaN (gallium aluminum nitride) semiconductor materials in particular are actively conducted lately.
Still more, FETs (Field Effect Transistor) using a MIS (Metal Insulator Semiconductor) structure for a gate structure of a transistor using the GaN/AlGaN semiconductor material are being developed lately for the purpose of reducing a gate leak current as disclosed in Japanese Patent Application Laid-Open No. Hei.11-261052, for example.
[Patent Document 1] Japanese Patent Application Laid-Open No. Hei.11-261052.
By the way, there is one having a HEMT (High Electron Mobility Transistor) structure as a lateral-type semiconductor element using the late GaN semiconductor material. The GaN semiconductor element having the HEMT structure (hereinafter referred to simply as a GaN HEMT) can utilize two-dimensional electron gas that is generated around a hetero junction interface as carriers, so that it is capable of reducing on resistance. Still more, the GaN HEMT uses a schottky electrode for its gate structure, so that it is capable of enhancing its withstanding characteristic. It is noted that the on resistance described above refers to a resistance between a source and a drain during operation.
However, due to a structure of the schottky electrode, its gate leak current is greater than that of the semiconductor element having the MIS structure, so that there has been a problem that it is difficult to use the GaN HEMT for a semiconductor element that is required to have a relatively high withstanding characteristic.
Still more, the GaN HEMT has a problem that a drain current remarkably reduces when an influence of a current collapse between the gate and the drain increases and the on resistance increases.
It is conceivable to apply the MIS structure disclosed by Japanese Patent Application Laid-Open No. Hei.11-261052 described above to the GaN HEMT as a method for solving such a problem. However, the structure disclosed by Japanese Patent Application Laid-Open No. Hei.11-261052 has not fully improved the influence of the current collapse between the gate and the drain. Due to that, it has been unable to realize a degree of high withstanding characteristic and low-on resistance required by late power transistors and others by the conventional GaN HEMT.